Although many lines of evidence document the importance of maintaining a relatively high calcium intake during pubertal development, the calcium intake of as many as 20% of pubertal American girls is less than one-third of the currently recommended level. Virtually no information is available regarding the physiological adaptation to these low intakes. For example, despite a lower average calcium intake, peak bone mass is greater and the risk of osteoporosis is lower in African-American compared with Caucasian girls. This proposal will evaluate the adaptation to low calcium intakes by girls during puberty and compare the differences in that adaptation between Caucasian and African-American girls. The specific aims of the project have not changed during the initial year since the project began. To summarize, these were to: 1) To assess the effects of decreasing calcium intake to 400 mg/day in girls on calcium absorption, excretion and bone formation/resorption; 2)To determine the changes in calcium retention and kinetics after supplementing the intake of girls with very low habitual intakes of calcium (Group 2) with 600 mg of calcium for 6 months; 3) To evaluate racial differences in adaptation to low calcium intakes which may partially account for the lower incidence of osteoporosis in African-Americans compared to Caucasians despite the lower calcium intake of African-Americans; 4) To determine the relationship between hormonal and pubertal status and adaptation to low calcium intake in healthy girls.